Nuclear pores serve as the sole gates mediating nucleocytoplasmic molecular communication. They constantly accept heavy molecular traffic at a rate of ~1000 molecules per second, selected from a vast number of molecules randomly approaching the pores. The central channel of the pores are highly crowded with an intrinsically disordered region of pore-forming subunits and this channel functions as a selective permeability barrier. Recently, the phase separation properties of the hydrophobic subunits of pores have been reported, together with the flexible amphiphilic nature of the transporting molecules. These findings suggest that phase separation is a fundamental mechanism of action in nuclear pores. In this chapter, the entire nucleocytoplasmic transport system and composition of the nuclear pore complex are reviewed, followed by a detailed review of recent studies focusing on the characteristic features of both nuclear pores and transporting molecules. Finally, intrinsic and extrinsic factors that adaptively affect the function of the molecular crowding barrier are introduced.